1. Technical Field
The present invention relates to a motion picture coding apparatus, and, more particularly, to a motion picture coding apparatus for use in a system, which transmits motion pictures over a line, such as a video conference or a video telephone, a system for storing motion pictures on a storage medium, such as an optical disk or a video tape, and digital television broadcasting.
2. Description of the Related Art
Today, standardization for a scheme for transmitting motion pictures, which has high quality equal to or higher than the quality for the current TV system, at a rate of about several Mbps to several tens of Mbps has progressed in the communication field for future establishment of the service in a broad-band network (H. 26X). Likewise, in the fields of the storage system and broadcasting system, standardization for a coding system which has about the same picture quality and bit rate as H. 26X has progressed (MPEG2 for the storage system and CMTT/2 for the broadcasting system). Since it is considered advantageous for popularization to establish systems as common as possible in those standardizations, the standardizations have progressed while mutually exchanging information.
In the communication system and storage system, there already are H. 261 and MPEG1, which are standard systems for motion picture coding whose targets are lower picture quality and lower bit rate. Specifically, the supply of hardware for those standardizations has started. Therefore, one of issues is that a new standard system to achieve higher picture quality should have mutual connectability (compatibility) with those existing standard systems. There are two types of compatibility: forward compatibility which permits a decoder of the new system to decode a bit stream prepared by an encoder of an existing system and backward compatibility which permits a decoder of the existing system to decode a part of a bit stream prepared by an encoder of the new system. In consideration of the fact that the decoder of the existing system has already been defined whereas the decoder of the new system has been undefined, the backward compatibility would be a severe requirement for the coding system.
There may be various systems for ensuring the backward compatibility. As one of those methods that can define the compatibility as an option, a system has been proposed which has a local decoded signal of an existing system included as a candidate for a predictive signal using in the new system and forms a bit stream in such a way that a part of a bit stream by the new system becomes a bit stream by the existing system (which is called embedded coding using hierarchical coding).
Chapter 6 of "International Standardization of Multimedia Coding" (published by Maruzen) describes MPEG1, which is one of the existing systems. In that chapter, the section 6.3 "Coding Algorithm" describes an inter-picture predicting structure which consists of an I picture (intra-prediction picture), P picture (forward prediction picture) and B picture (both forward and backward prediction pictures), with reference to FIG. 6.2. In the description, it is expected that the new system also takes an inter-picture prediction structure similar to that of the existing system. While "picture" is a "frame" in the existing system, such as MPEG1, due to a picture signal having a non-interlaced structure, however, it may become "field" having an interlaced structure in the new system, making the prediction structure slightly complicated.
In the aforementioned embedded coding using hierarchical coding, prediction using a local decoded signal by the existing system is added as an option, and this prediction is selected only when it is better than the prediction originated from a local decoded signal by the new system. In other words, a better one of a local decoded signal from a local decoder of the new system and a local decoded signal that is input via an up-sampling circuit from a local decoder of the existing system is selectively used as a predictive signal in a coding section. It is therefore considered that the coding efficiency will not be deteriorated by the inclusion of the latter prediction from the local decoded signal by the existing system. Conventionally, however, only the prediction error power is considered in the mode discrimination for selecting a predictive signal used in the coding section and that predictive signal which has smaller prediction error power is selected, so that the prediction using the local decoded signal of the existing system is not sufficiently utilized. That is, while the amount of information generated at the time of coding a prediction error signal is reduced, the amount of information as the entire coded output including motion vector information should not necessarily be reduced. This stands in the way of improving the coding efficiency.
For the I picture among individual pictures classified by the prediction structure, the prediction by the existing system is effective and will be selected accordingly, whereas for the P picture and B picture, the prediction originating from a local decoded signal for the coding result by the existing system is not so effective and the chance of its being selected becomes very low. It is known that one cause for this depends on the way of preparing a picture to be coded by the existing system.
Further, according to the conventional embedded coding using the hierarchical coding, in the case where a low-resolution picture is prepared by dropping one field off, the field that has not been used in preparing the low-resolution picture will not be well predicted by a low-resolution predictive signal, thus lowering the entire prediction efficiency.
As described above, according to the conventional embedded coding using the hierarchical coding, since only the prediction error power is considered in the mode discrimination for selecting a predictive signal in the coding by the new system, the prediction using the local decoded signal of the existing system cannot be utilized sufficiently, which stands in the way of improving the coding efficiency.
In addition, according to the conventional embedded coding using the hierarchical coding, for the I picture among individual pictures classified by the prediction structure, the prediction by the existing system is effective and will be selected accordingly, whereas for the P picture and B picture, the prediction originating from a local decoded signal by the existing system is not so effective and the chance of its being selected becomes very low.